The present disclosure relates a cascode-type transconductance amplifier which is capable of reducing distortion and a variable gain circuit which is capable of reducing distortion and noise.
Tuner systems that receive transmitted signals each containing multiple channels and that select and demodulate a desired one of the multiple channels are required to have low-noise and low-distortion characteristics. For example, the Integrated Service Digital Broadcasting-Terrestrial (ISDB-T) in Japan has 40 channels, in total, from Channel 13 (473.143 MHz) to Channel 52 (707.143 MHz), where each channel has a signal band of 6 MHz. A tuner system is required to realize high sensitivity of −80 dBm or less and immunity of 50 dBc or more with respect to the input levels of interfering waves.
The reception characteristics of a tuner system of this type depend on the noise characteristics and distortion characteristics of a variable gain circuit that processes RF signals that have been just received by an antenna, for example. On the other hand, a tuner system installed in a mobile terminal is required to reduce its power consumption.
A variable gain circuit configured to process an RF signal in two paths in a switchable manner depending on the signal strength is disclosed (see, for example, U.S. Pat. No. 8,018,285). Specifically, an RF signal having a low signal strength is amplified by a transconductance amplifier provided in a first path. On the other hand, an RF signal having a high signal strength is attenuated by an attenuator provided in a second path, and then amplified by a transconductance amplifier provided in the second path.
With this configuration in which the first path is turned on and the second path is turned off if an RF signal has a low signal strength, and the first path is turned off and the second path is turned on if an RF signal has a high signal strength, an attempt to reduce noise and distortion has been carried out.